linux-kernel - [tip:locking/urgent] futex: Mention key referencing differences between shared and private futexes

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Date:	Sun, 26 Oct 2014 08:22:28 -0700
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	dbueso@...e.de, peterz@...radead.org, hpa@...or.com,
	Matteo.Franchin@....com, linux-kernel@...r.kernel.org,
	dave@...olabs.net, catalin.marinas@....com,
	torvalds@...ux-foundation.org, dvhart@...ux.intel.com,
	paulmck@...ux.vnet.ibm.com, davidlohr@...com, mingo@...nel.org,
	tglx@...utronix.de
Subject: [tip:locking/urgent] futex:
  Mention key referencing differences between shared and private futexes

Commit-ID:  993b2ff221999066fcff231590593d0b98f45d32
Gitweb:     http://git.kernel.org/tip/993b2ff221999066fcff231590593d0b98f45d32
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Thu, 23 Oct 2014 20:27:00 -0700
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Sun, 26 Oct 2014 16:16:18 +0100

futex: Mention key referencing differences between shared and private futexes

Update our documentation as of fix 76835b0ebf8 (futex: Ensure
get_futex_key_refs() always implies a barrier). Explicitly
state that we don't do key referencing for private futexes.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Cc: Matteo Franchin <Matteo.Franchin@....com>
Cc: Davidlohr Bueso <davidlohr@...com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Darren Hart <dvhart@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Catalin Marinas <catalin.marinas@....com>
Link: http://lkml.kernel.org/r/1414121220.817.0.camel@linux-t7sj.site
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/futex.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index f3a3a07..bbf071f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -143,9 +143,8 @@
  *
  * Where (A) orders the waiters increment and the futex value read through
  * atomic operations (see hb_waiters_inc) and where (B) orders the write
- * to futex and the waiters read -- this is done by the barriers in
- * get_futex_key_refs(), through either ihold or atomic_inc, depending on the
- * futex type.
+ * to futex and the waiters read -- this is done by the barriers for both
+ * shared and private futexes in get_futex_key_refs().
  *
  * This yields the following case (where X:=waiters, Y:=futex):
  *
@@ -344,13 +343,20 @@ static void get_futex_key_refs(union futex_key *key)
 		futex_get_mm(key); /* implies MB (B) */
 		break;
 	default:
+		/*
+		 * Private futexes do not hold reference on an inode or
+		 * mm, therefore the only purpose of calling get_futex_key_refs
+		 * is because we need the barrier for the lockless waiter check.
+		 */
 		smp_mb(); /* explicit MB (B) */
 	}
 }
 
 /*
  * Drop a reference to the resource addressed by a key.
- * The hash bucket spinlock must not be held.
+ * The hash bucket spinlock must not be held. This is
+ * a no-op for private futexes, see comment in the get
+ * counterpart.
  */
 static void drop_futex_key_refs(union futex_key *key)
 {
--
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